The present invention relates to an improved high pressure low volume pump suitable for use in high pressure liquid chromatography.
There is a need for a pump that can accurately deliver precisely measured, very small volumes of liquid at very high pressures. For example, in performing high pressure liquid chromatography (HPLC) procedures, a motor driven pump is typically used to deliver liquid solvents such as methanol, isopropyl alcohol and the like. The trend is to use smaller volumes of solvent for the mobile phase of the chromatography column and to operate at higher pressures. For example, it would be desirable to provide a pump that can deliver fluids at low flow rates in the range of from about 50 nanoliters to about 250 microliters per minute at pressures of several hundred bars.
A piston pump designed for such low flow volumes is necessarily delicate because the liquid handling components of the pump must be very small in size. Low volume HPLC pumps can benefit from the use of a small diameter piston made of sapphire or zircon or the like, because such materials can be provided to close dimensional and surface tolerances in very small sizes. However a problem exists because this material is fragile and easily broken. It is difficult to avoid breakage of a small and delicate piston during assembly and operation of the high pressure low volume pump.
A principal object of the present invention is to provide an improved high pressure low volume pump capable of providing accurately metered flows of liquids in the nanoliters per minute range at pressures as high as several hundred bars. Further objects are to provide a pump that can employ a very small piston made of a fragile material while overcoming the problem of breakage of the piston during assembly and operation of the pump; to provide a pump in which the need for mechanical piston retention, for example by a spring, is avoided; to provide a pump which does not require precise and expensive alignment of the piston with the piston drive system; and to provide a high pressure low volume pump overcoming the disadvantages of pumps that have been used in the past.
In brief, in accordance with the invention there is provided a high pressure low volume pump for high pressure liquid chromatography and the like. The pump includes a pumping section including a pump cylinder and passages for the flow of a pumped fluid into and out of the cylinder. A piston assembly includes a piston reciprocally movable in the cylinder and a piston holder supporting the piston at a first end of the piston holder. A piston drive system is connected between a motor and the second end of the piston holder for reciprocating the piston assembly in response to operation of the motor. The piston is an elongated slender rod having a diameter of less than about 10 millimeters. The interconnection of the drive system and the second end of the piston holder includes a ball-and-socket coupling with a spherical member pivotally received in a socket. A magnet in the socket holds the spherical member in the socket using magnetic force.